Prevention of h2o invasion of in situ combustion zones



Jan. 16, 1962 J. w. MARX 3,016,953

PREVENTION OF H2O INVASION OF IN SITU COMBUSTION ZONES Filed June 23, 1958 4 AIR PRODUCTION AIR l ill l8 22 1 t H2O [I6 VENT 2O 32 34 WELL INVENTOR. J. W. MARX A TTORNEKS Unite atent fiice 3,016,953 PREVENTION GF H 9 INVAMON OF IN SITU IOMIBUSTION ZONES John W. Marx, Bartiesville, Ulrla, assignor to Phillips Petroleum ornpany, a corporation of Delaware Filed June 23 1958, Ser. No. 743,695 Claims. (Cl. 16611) This invention relates to a process for recovering hydrocarbons from a carbonaceous stratum by in situ combustion. A specific aspect of the invention pertains to the prevention of H 0 invasion of a carbonaceous stratum being produced by in situ combustion.

In situ combustion in the recovery of hydrocarbons from underground strata containing carbonaceous material is becoming more prevalent in the petroleum industry. In this technique of production, combustion is initiated in the carbonaceous stratum and the resulting combustion zone is caused to move through the stratum by either inverse or direct air drive whereby the heat of combustion of a substantial proportion of the hydrocarbon in the stratum drives out and usually upgrades a substantial proportion of the unburned hydrocarbon material.

The ignition of carbonaceous material in a stratum around a borehole therein followed by injection of air through the ignition borehole and recovery of product hydrocarbons and combustion gas through another borehole in the stratum is a direct air drive process for effecting in situ combustion and recovery of hydrocarbons from the stratum. In this type of operation the stratum usually plugs in front of the combustion zone because a heavy viscous liquid bank of hydrocarbon collects in the stratum in advance of the combustion zone which prevents movement of air to the combustion process. To overcome this difficulty and to permit the continued progress of the combustion zone through the stratum, inverse air injection has been resorted to. By this technique, a combustion zone is established around an ignition borehole by any suitable means and air is fed through the stratum to the combustion zone from one or more surrounding boreholes.

In situ combustion techniques are being applied to tar sands, shale, Athabasca sand, and other strata in virgin state, to coal veins (by fracturing), and to strata partially depleted by primary and even secondary and tertiary recovery methods.

Field tests indicate that extensive vertical and horizontal fracturing may occur in formations as an underground combustion front spreads. Some of these vertical fractures extend into overlying and underlying water zones, permitting water to invade the burning formation and quench the fire. The vertical fractures tend to let air by-pass the pay zone, and also to drive water into same which may result in quenching of the flame. Water invasion of the pay zone at the bottom is less serious than water invasion at the top.

In some cases the water invasion can be halted by increasing the back pressure in the formation to more than the hydrostatic pressure. However, field test results show that this does not always work. Where this fails, it is believed that air from the injection wells has entered the water zones, and this overlying and underlying water is being driven, at injection pressures, into the pay zone, Where it tends to put out the fire.

Accordingly it is an object of the invention to provide a process or method of preventing the invasion of a carbonaceous stratum undergoing in situ combustion by water from an adjacent water-containing formation, such as a permeable water sand. Another object is to provide a process for the recovery of hydrocarbons from a carbonaceous stratum by in situ combustion which prevents H O invasion of the producing stratum from an adjacent watercontaining formation during the process. Other objects of the invention will become apparent upon consideration of the accompanying disclosure.

A broad aspect of the invention comprises the improvement in an in situ combustion process wherein a combustion front is moved through a carbonaceous stratum by igniting same and injecting O -containing gas into the ignited area and produced hydrocarbons are driven from the stratum in vapor form, which improvement comprises drilling at least one vent well into the adjacent watercontaining structure to permit gas pressure within the stratum to force Water into said vent well, thereby reducing flow of water into the stratum. The invention is particularly adapted to structures where the carbonaceous stratum being produced lies close to a permeable water sand and is separated therefrom by a thin impermeable rock layer. During the in situ combustion process the stratum being produced reaches a temperature in the range of 1000 to about 1700 F. thereby creating fractures in the stratum itself and, oftentimes, in the adjacent impermeable rock layer. Some of the fractures in the adjacent rock layer allow gas to escape into the water sand, thereby forcing water through other fractures into the stratum being produced. This invasion of water results in extinguishment of the fire and termination of the in situ combustion process, in many cases. By drilling a borehole into the water sand the pressure of the escaping gases and hydrostatic pressure in the sand force water into the vent borehole from which it is readily removed by pumping or by forced fiow.

More complete understanding of the invention may be had by the reference to the accompanying schematic drawing which is an elevation in partial section of an arrangement of boreholes and apparatus in an earth structure including a carbonaceous stratum and adjacent permeable Water sands for effecting the invention.

Referring to the drawing, a carbonaceous stratum 10 is penetrated by an ignition and production borehole 12 and by air injection boreholes 14. Ignition borehole 12 is provided with casing 16 and tubing 18, while injection boreholes 14 are provided with casing 20 and tubing 22. Carbonaceous stratum 10 lies adjacent a relatively thin cap rock 24 and also adjacent a relatively thin impermeable rock substratum 26. Directly above and below the impermeable rock layers bounding stratum 1.0 lie permeable water sands 28 and 30, respectively. A vent well 32 is drilled from the surface into the permeable sand 28 and another vent well 34 is drilled into Water sand 30, lying underneath the carbonaceous stratum 1t]. Vent well 34 should be sealed in rock layers 24 and 26 as by cementing as illustrated at 36, in order to minimize the flow of Water from sands 28 and 30, respectively, into stratum 10.

In situ combustion front 38 is driven through stratum 10 in an inverse air injection technique from ignition and production well 12 to injection wells 14 by initiating combustion in the stratum adjacent well 12 by conventional means and injecting air or other O -containing combustion-supporting gas through wells 14 so as to feed the hot combustion zone around well 12 and cause the combustion front to move through the stratum countercurrently to the flow of air. It is also feasible to drive the combustion front through the stratum between well 12 and wells 14 by direct air drive through borehole 12, in applications in which the carbonaceous stratum is suitable for direct air drive.

As the combustion zone 38 moves out from well 12 the stratum behind the combustion front is heating to a temperature in the range of about 1000 to 1700" F., particularly in the inverse air injection technique. This heating of the stratum to high temperature usually cracks or fractures the stratum so that these cracks or fractures extend into and, usually, through adjacent rock layers 24 and 26, which are also substantially heated by the combustion process. These cracks illustrated at 40 sometimes extend completely through the impermeable rock layers and allow the gas behind the combustion front to escape into the water sand thereby increasing the pressure therein and driving some of the water back into stratum through other cracks in the separating rock layer. By positioning vent wells 32 and 34 in the water sands, pressure developed therein by the water head and by the gas pressure in stratum 1t forces water out the vent wells and thereby reduces the tendency of the water to flow into stratum 10. Of course, it is also helpful to pump these venting wells during the operation.

The drawing can be interpreted as representing either a ring type production pattern in which a central well 12 is surrounded by a ring of injection wells 14 in a 5, 7, or 9-spot conventional Well pattern, or a line of ignition wells 12 flanked on each side by a generally parallel line of injection wells 14. It is to be understood that vent Wells 32 and 36 may represent a ring or line of these wells in these two types of recovery patterns.

Certain modifications of the invention will become apparent to those skilled in the art and the illustrative details disclosed are not to be construed as imposing unnecessary limitations on the invention.

I claim:

1. In a process for producing hydrocarbons from a permeable carbonaceous stratum separated from a permeable water sand by an impermeable layer by in sit'n combustion comprising igniting said stratum adjacent an ignition well therein; and driving the resulting combustion front through said stratum to an offset well by feeding combustion supporting gas to said front through one of said wells and producing hydrocarbons through the other well, wherein said impermeable layer develops cracks from heat of combustion and subjects said stratum to flooding, the improvement comprising drilling at least one vent well into said water sand intermediate said ignition and offset wells, said vent well being closed to said stratum, thereby permitting gas pressure developed within said stratum by injection and combustion to pass through said cracks and force water into said vent well and reduce the flow of water into said stratum.

2. The process of claim 1 wherein said water sand is above said stratum.

3. The process of claim 1 wherein said water sand is below said stratum.

4. The process of claim 1 wherein said combustion front is being moved from a central well to a ring of surrounding Wells and plurality of vent wells are drilled intermediate said central well and said ring of wells.

5. The process of claim 1 wherein said combustion front is being moved from a line of ignition wells to an adjacent parallel line of wells and vent wells are drilled intermediate the lines of Wells.

References Cited in the file of this patent UNITED STATES PATENTS 2,584,605 Merriam et al Feb. 5, 1952 2,734,579 Elkins Feb. 14, 1956 2,788,071 Pelzer Apr. 9, 1957 t l I 

1. IN A PROCESS FOR PRODUCING HYDROCARBONS FROM A PERMEABLE CARBONACEOUS STRATUM SEPARATED FROM A PERMEABLE WATER SAND BY AN IMPERMEABLE LAYER BY IN SITU COMBUSTION COMPRISING IGNITING SAID STRATUM ADJACENT AN IGNITION WELL THEREIN; AND DRIVING THE RESULTING COMBUSTION FRONT THROUGH SAID STRATUM TO AN OFFSET WELL BY FEEDING COMBUSTION SUPPORTING GAS TO SAID FRONT THROUGH ONE OF SAID WELLS AND PRODUCING HYDROCARBONS THROUGH THE OTHER WELL, WHEREIN SAID IMPERMEABLE LAYER DEVELOPS CRACKS FROM HEAT OF COMBUSTION AND SUBJECTS SAID STRATUM TO FLOODING, THE IMPROVEMENT COMPRISING DRILLING AT LEAST ONE VENT WELL INTO SAID WATER SAND INTERMEDIATE SAID IGNITION AND OFFSET WELLS, SAID VENT WELL BEING CLOSED TO SAID STRATUM, THEREBY PERMITTING GAS PRESSURE DEVELOPED WITHIN SAID STRATUM BY IMJECTION AND COMBUSTION TO PASS THROUGH SAID CRACKS AND FORCE WATER INTO SAID VENT WELL AND REDUCE THE FLOW OF WATER INTO SAID STRATUM. 